drilled shaft pier vs helical

[rgensler]

We are planning a 400 sq foot deck attached to a house that has a hillside out the back. Our geo-eng. spec'd 30" piers 12' into the bedrock. The total pier length would be 27'. The deck would require 4 of these. I have inquired about helical piers. He said they are not for use with lateral loads. Although, I have found a company that manufactures a LRD for use with a helical. The house was built in 1959 and has some type of piering system although I doubt 12' into the bedrock. We want something that is adequate but don't want to install something that is more than we need. Appreciate any advice.

 

[ishvaaag]

The question is that reliability according to the code with a hill behind of doubtful behaviour may require according to modern practice such big sizings; this comes of the adaptation by modern foundations' requirements of the ability to stand the famous 95% of the times the worst scenario case it may develop, and geotechs are knowledgeable and very wary (then) of what a worst scenario case is.

Contrarily, the 50's design of foundations was more customary and in accord to what the average behaviour is. Obviously, the average behaviour is far far from the development of a worst case scenario. As far, for example, as the normal operation of the hotel was from plunging to the river in the typhoon of two days ago. Imagining the hill sliding past your socketed piers like butter, er, makes one willing to search for another site, and this is one thing an owner wanting to have some house there is not surely wanting to consider. So the practical approach is go with the recommendations of the geotech, you'll be in the warrant of their recommendation and in the end it may turn reasonable in current practice terms.

 

[PSlem]

Four 27' 30" piers 12' in rock for a deck? Engineering is supposed to be about doing with $1 what anyone can do with $2.
If worst case happens, is house founded on similar foundations? It's been fine for 60 years.

 

[rgensler]

Unknown foundation of the house. It sits uphill from the proposed deck. We dug down a few feet around the slab and found some type of footings that went deeper.

My point to the geo-tech is that when the big one hit I won't be standing on my deck because it will have been crushed by the collapse of the house. We had a structural engineer take a look at the house before we purchased it and he found no issues with it.

Thanks for your input.

 

[GeoPaveTraffic]

The diameter of the shafts is larger than I would expect, 12 or 18 inch should be fine for a deck. As for 12 feet into bedrock for a deck, a blind monkey could have come up with a better design than that. Depending on the amount of wethering, a couple of feet should be plenty.

The number of required piers is related the way the deck if framed. By using larger framing members, you could reduce the number to a minimum of 2.

 

[GeoPaveTraffic]

Forgot to comment on the helical anchors. If you can get them anchored into the rock, they might work. However, they generally have very little lateral support capability.

 

[rgensler]

Our architect specified 18" diameter 12" into the bedrock for the piers. We thought the 12' specified into the bedrock by the geo-tech might have been a typo, but unfortunately it is not a typo. One more bit of data. The slope of the bedrock is 17 degrees with approximately 15 feet of soil on top.

Any ballpark figures on what these four drilled shaft piers would cost to install? I have a drilling company coming out next week to give me a proposal but I don't want to waste more time if the cost is going to be prohibitive.

 

[rchanke]

You may start thinking about what type / size of equipment would be used to place these drilled piers in rock. Regardless of the competency of the rock, 30" diameter will require a large drill rig which may very well lead to access issues if working on a tight slope behind your house.

I'd ask your geotech for alternates to what he proposed initially. Perhaps, in place of 4 large piers a larger number of smaller diameter micropiles could satisfy. Micropiles (5" to 7" dia.) placed both vertically and in a battered array will carry the lateral load of a deck and can also be keyed into rock with relatively small sized equipment when compared to drilled pier machines.

Helicals can carry a lateral load when battered and tied to vertical units, but can not be keyed into rock.

 

[InDepth]

More than likely the geotech is trying to improve the site with the recommendation...prevent a slope instability failure.

The large diameter and embedment of the pile is probably based on a calculation assuming the pier behaves as a cantilever. If you tie-back the top, you may get a more reasonable size. Also, with this concept the required embedment should be much less.

One option that could be explored is whether you can use a helical anchor for vertical load and a second helical anchor on an angle to resist the lateral load. You could also try micropiles (1 vertical, and one at a 45 degeree angle). See attached. These system can serve dual purpose.

Another option is a drilled pier with a cap beam and a micropile or helcial tieback installed on an angle into the cap beam. Depending on your spacing of piles and cap beam I'm sure you could get 16"-18" piers.

 

[jhoulette]

I agree with rchanke, if you have limited access, you are stuck with small equipment and therefore small concrete foundation or helical piers.
Some helical pier manufactures have caps that fit on the helical pier shaft and provide for lateral stability, LRD or Lateral resistance device as you mentioned. That should be plenty of support.
For a deck, I'm curious what lateral loads are being braced by a 30" diameter pier? The wood posts don't transfer heavy lateral loads.
I do understand why 12ft into bedrock was specified, due to very small deadload to resist the uplift pressure on the pier, therefore deeper bedrock penetration. But again, this is a deck, some movement can be expected.
Why not just use 18" x 18" x frost depth pads? The house was built so long ago that the soil has been exposed to the additional moisture from lawns and such that any swelling or settlement has taken place. Any future movement should only be from year to year variations in moisture.

 

[palmahouse]

Be sure to evaluate corrosion potential for the helicals.